The invention relates to a device for positioning the measuring element of measuring instruments.
Such devices are for example used to determine and monitor the thickness of plastic in the production of these films in relation to the goods to be measured. Plastics are for example extruded in a tubular shape and blown to form a bubble whose circumference corresponds to the width of the film to be produced. This method is known as blown film extrusion. When viewed in the direction of movement of the bubble, just downstream of the extruder, an arcuate or annular track is arranged around the plastic film bubble, as concentrically as possible in relation to the bubble made of plastic film. On this track a traverse is arranged, e.g. on pivoted bolsters, on which traverse the measuring element or the measuring head and as a rule also other components of the measuring apparatus are installed. By moving the pivoted bolsters on the track the measuring head with the measuring element can be positioned and guided along the track at adjustable spacing. In a circular track this means that the measuring element is positioned and guided radially and angularly. In this way it is possible to position the measuring head in a radially correct manner in relation to the goods to be measured, e.g. the film bubble, and to guide the measuring head, e.g. at the correct spacing, around the circumference of the goods to be measured.
The measuring element of the measuring apparatus monitors the thickness of a plastic film only along a width of a few millimeters or centimeters. As a rule, in the direction of movement of the extruded film, i.e. in the production direction, the thickness of plastic film varies only slightly rather than varying suddenly and abruptly. Variations in the thickness of film around the circumference of an extruded film bubble are experienced significantly more frequently than are variations along the film bubble. Thus, predominantly variations in thickness transverse to the direction of movement of the extruded film are experienced, in the so-called transverse profile. In most cases it is thus sufficient to move the measuring head e.g. periodically around the circumference of the film bubble in order to detect faults in the thickness of the bubble. Moving the measuring head to and fro on the annular track around the extruded bubble is referred to as “reversing”. However, the measuring head can also circle continuously around the film bubble, on the annular track.
This movement to and fro of the measuring apparatus around the circumference of the bubble or the motion of rotation around the bubble can be achieved in various ways. For example a trolley can be arranged on the rail-like track, with the measuring unit being attached to the trolley. The trolley can be self-propelled or driven in some other way, e.g. using a continuous rope or pulley. Flexible cables or sliding contacts such as slip rings can be provided for transmitting the measuring signals, control signals and/or the supply of power to the measuring unit and the drive. Measuring signals and control signals can also be transmitted wirelessly. EP 1 116 931 A1 describes a device for positioning measuring elements, in which device a rigid traverse is slidably held by two pivoted bolsters, which are for example driven, and is carried by the pivoted bolsters. The pivoted bolsters run on a rail track, which is for example circular, and can move on this track towards each other, away from each other or in unison with each other. In the case of identical movement of the two pivoted bolsters the traverse and in particular the measuring element move in a circular track on the traverse. By sliding the pivoted bolsters towards each other or away from each other the radius of the circular track on which the traverse and the measuring element move is changed.
For example in a circular rail track the length of the traverse is determined by the radial region in which the measuring element is to be positioned. This means that in the case of positions of the measuring element involving large radii, i.e. near the rail track, the ends of the traverse protrude beyond the rail track. This requires further space for the positioning device. In many cases this additionally required space simply does not exist.
It is also possible to do without the traverse. In such a solution a radially aligned telescopic rod is installed on a trolley. The measuring element is arranged on the telescopic rod and is radially slidable and positionable with the telescopic rod. Such a solution is not very stable at a large adjustment range of 1 m to 3 m. There is the danger of undesirable oscillation occurring. In the case of long lengths of the telescopic rod, the torque exerted by the telescopic rod on the trolley becomes very significant. Such deficiencies can only be remedied with very considerable construction expenditure, which for economic reasons is usually not justifiable.